1. Field of the Invention
The invention relates to a device for producing an electron beam. In particular, a “slimmer” electron beam with a small focal diameter and a high power density may be produced with a device according to the invention and may act at a defined process location inside a vacuum chamber, wherein the beam generator and power supply assemblies thereof are characterized by a compact structure, ease of maintenance and relatively low manufacturing costs. Typical technological application fields for an electron beam generator according to the invention are the coating of substrates with functional layers (such as, e.g., layers for corrosion protection, decoration, diffusion barriers, EMC shielding, thermal insulation, etc.) using physical vapor deposition (PVD), the cleaning and remelt processing of metals in a vacuum as well as the adhesive joining of components by fusion welding.
2. Discussion of Background Information
The processing of semi-finished products and products using decorative or functional thin-film systems is an important work field in surface engineering. In this case, the versatile, environmental-friendly and economical methods of physical vapor deposition (PVD) have found wide-spread use in production processes, in which the coating material is first vaporized in a vacuum and then condensed as a film on the respective substrates in a controlled manner, sometimes also with the addition of reactive gases. In addition to the achievable morphology, purity and adhesion of the layer being deposited, important criteria for the selection of a suitable coating method from the different technologies available are above all also its build-up speed as well as investment needs and operating costs for the vaporizer, because the economy of the process depends significantly on these characteristics.
Electron beam sources of different designs have been used for many decades for implementing industrial high-rate PVD methods. Electron beam vaporizers supply the greatest commercially established coating rates with simultaneously excellent uniformity and purity of the deposited layer, and this also in the case of reactive, ferromagnetic and high-melting coating materials. These characteristics result from the high power flux density that can be adjusted inertia-free by magnetic focusing and beam guidance as well as the direct heating of the vapor-emitting surface. As a result, the crucibles required for storing the coating material may be cooled, and thus, do not cause any contamination of the coating.
Cathodes heated exclusively to a high operating temperature are currently established as electron sources for industrial PVD processes, in which the production of free electrons is based on the thermoelectric effect (GB 1 041 282 A). The functional principle of these electrodes, also called “thermionic cathodes,” causes the traditional electron emitters to be more structurally complicated and be designed in a relatively involved manner with respect to their power supply devices and that certain embodiments are only able to cover a very limited range of technological applications.
For example, so-called transverse electron beam sources (also called “transverse EB guns”), in which the beam generation, magnetic 270° beam deflection and a crucible with vaporizer material are integrated most of the time in a compact functional block, are a common vapor source for the electron beam vaporization (also called “EB PVD”). These sources are relatively inexpensive, but are limited in terms of their maximum beam power (approximately 20 kW) as well as acceleration voltage (approximately 20 kV), and are therefore, also limited with respect to the vaporization rate that can be produced. In addition, the actual beam source (cathode with heating device) is located at the pressure level of the coating chamber and is directly exposed to the vapors and gases located therein.
Consequently, the pressure in the coating chamber must be kept at low values by correspondingly generous dimensioning of the vacuum pumps in order to prevent instabilities when operating the electron source. In the case of high-rate deposition of dielectric compounds, which, to ensure a stoichiometry that meets requirements, requires a reactive process management, i.e., the setting of a relatively high partial pressure (approximately 0.1 Pa to 1.0 Pa) of reactive gases inside the vacuum chamber, “transverse EB guns” have not been able to gain acceptance despite numerous structural or circuit-related improvements, especially due to their unacceptably high tendency toward high-voltage spark-overs under these process conditions.
So-called axial electron beam sources (“axial EB guns”) that are designed for vaporization methods with beam powers up to 300 kW and acceleration voltages up to 60 kV (for special applications also up to 75 kV) are a technologically more powerful beam tool for EB PVD. The cathode chamber of these types of beam generators is separated from the process chamber by panels with a small mostly circular opening for the passage of the beam, which function in terms of the vacuum as flow resistances, and is evacuated separately with additional high-vacuum pumps (in contemporary embodiments by turbomolecular pumps). Therefore, the vaporization process is also still able to run at higher pressures and especially also with a high proportion of reactive gases in the coating chamber. Furthermore, higher coating rates are achieved hereby without losses in stability. However, these types of systems are quite expensive with respect to the required investment costs and for economic reasons are therefore also able to be used advantageously only in a narrow field of application.
In order to overcome this limitation, several cold-cathode beam generators with a plasma anode were proposed in which the electron release is not based on the thermoelectric effect, but results from the ion bombardment of a large-scale metal electrode. A high-voltage glow discharge maintained in the beam source hereby produces ions and accelerates said ions to the cathode. The electrons transferred there ballistically from the solid body into the vacuum are accelerated in the cathode fall of the plasma and formed by suitable electrode contours into a homocentric beam, which is able to be focused with conventional electron-optical assemblies and guided to a vaporizer.
While thermoelectric emitters require a high vacuum of greater than 10−3 Pa in the cathode chamber, the operating pressure with a plasma-stimulated cold cathode is in the range of 2 to 10 Pa depending upon the operating voltage, plasma process gas, and the discharge current that is currently in demand. Therefore, it is possible to dispense with a differential evacuation of the beam source up to a pressure of approximately 1 Pa in the coating chamber without sacrificing the essential advantages of the axial emitter such as technological universality, as well as spatial and vacuum-related separation of the vaporizer and the beam source, and the associated gain in reliability. The regulation of the beam power takes place in this case by varying the plasma density in the cathode chamber by a rapid gas flow regulation. Instead of the previously common multi-conductor high-voltage feed in the case of thermionic cathodes, a single-pole cable suffices, and the high-voltage power supply also does not require any additional power supply unit floating at a high potential. It must be emphasized as the economically significant result that systems realized on the basis of cold-cathode emitters, which are made up of a beam source including power supply and control components thereof, are able to be manufactured at considerably lower costs as compared to conventional axial emitter systems.
The described cold-cathode axial emitters thus have many advantages as compared to conventional thermionic cathode emitters, but they also have some inadequacies in the case of certain technical parameters or for special applications. Thus, it requires the relatively low achievable emission current density of the cold cathode (100 mA/cm2 as compared up to 10 A/cm2 for a tungsten thermionic cathode) to implement large-scale cathodes for high currents. This results in a tendency for a larger beam diameter and a lower power density at the process location. The physical size of the beam source therefore increases again in an undesirable manner in the high-performance range. In addition, the electron optics are more expensive, while in general the vaporization rate is somewhat lower than with conventional systems with the same nominal power.
The process gas required to maintain the high-voltage glow discharge in the cathode chamber flows permanently through the axial opening required for outcoupling the beam into the process chamber, because, to regulate the discharge in the emitter itself, a slight excess pressure must always be maintained as compared to the process chamber. This gas load must be pumped off by the vacuum system of the process chamber in addition to the technologically induced incidence of gas.
Moreover, in order to ensure an acceptable degree of efficiency (relationship between the beam power that can be outcoupled and the total power supplied to the discharge, target value: >90%), reactive components such as, for example, oxygen or carbon dioxide, are required in the plasma process gas. Said reactive components serve to form and stabilize dielectric coats on the cathode surface and therefore to increase the secondary electron yield thereof (emitted electrons per incident ion). Viewed technologically, however, this approach is problematic for processes with high inertization or purity requirements, and, in terms of the system, it entails an additional effort when handling the significantly increased rate of high-voltage spark-overs as compared to metallically blank electrode surfaces.
The competing requirements for field strength relief for the cathode (great electrode distances are sought for this) and reliable dark-field shielding (small electrode distances are advantageous for this) make it more and more difficult to maintain high acceleration voltages in a stable manner in the long term in the case of plasma-based beam sources with increasing operating pressure. So far, operating voltages around 30 kV that have been dominated in the high-performance range with cold cathode emitters are sufficient for high-vacuum coating methods such as, e.g., metallizing (with approximately 0.001 to 0.01 Pa). For reactive high-rate coating processes with a typically distinctly higher pressure in the process chamber (approximately 0.1 to 1 Pa), however, voltages in the range of 40 to 60 kV would be more expedient due to the therewith better energy transport ability of the beam.
The power loss at the cathode resulting from the ion bombardment is relatively high in terms of order of magnitude at approximately 5% of the outcoupled beam power. A direct water cooling of the cathode is therefore indispensable with high beam powers. However, this is disadvantageous for two reasons. For one thing, the cooling water comes into contact with the electrode conducting the high voltage. Therefore, to reduce this potential difference with a low level of leakage current, as well as to ensure adequate personnel protection, several meters (reference value: >1 m/5 kV, respectively for forward and return path) of specially safety-insulated hoses must be laid. For another thing, the water circuit must be blocked, blown out, and opened in a labor-intensive manner for each cathode exchange.